Aluminum hydride, AlH3, (also referred to as alane) has long been known as a useful reducing agent in organic synthesis. With the advent of alternative energy technologies, alane has also shown great promise as a hydrogen storage material. Alane forms numerous polymorphs, the most thermally stable of which and the form most sought for hydrogen storage is α-alane, which has a cubic or rhombohedral crystalline morphology.
Hydrogen as an energy carrier can be implemented in a variety of devices operated by fuel cells, offering high energy density for portable power systems. A major drawback in its utilization has been the lack of acceptable hydrogen storage mediums. Conventionally, hydrogen has been stored in the gas phase under high pressure or in the liquid phase at extremely low temperatures. Unfortunately, such storage mechanisms require expensive processing and facilities (e.g., high pressure containers and low temperature maintenance). Alane, with a gravimetric capacity of approximately 10 wt. % hydrogen and a volumetric capacity of about 1.48 g/cm3, as well as an ability to release substantially all stored hydrogen effectively on demand, could be quite useful for solid phase storage of hydrogen for use as a fuel (for instance in a fuel cell application) and in solid energy applications (for instance as a propellant).
Unfortunately, alane remains untapped as a hydrogen storage material as the formation methods developed to date are economically unfeasible in the desired applications. The primary method currently used for synthesizing alane involves reacting aluminum chloride (AlCl3) and lithium aluminum hydride (LAIN in solution, generally utilizing diethyl ether as solvent. The alanate reactant of the process, LiAlH4, is very expensive, but use of lithium alanate as reagent has been necessary due to its solubility, which is necessary for the solution-based reaction to proceed.
Low yield has been another issue preventing economical adoption of alane for hydrogen storage applications. Alane monomer is thermodynamically unstable and as a result, in order to obtain significant alane product at all, it must be formed at high pressure or stabilized immediately upon formation. By use of a suitable electron donating solvent in the current processes (e.g., diethyl ether or tetrahydrofuran), alane adduct can form and stabilize the nascent monomeric product. However, this approach still leaves the problem of isolating the alane from other materials contained in the reaction mixture including impurities, excess reactants, and in some cases, the adduct complex partner itself, under conditions that polymerize the product in sufficient yield. For instance, the separation of alane from the ether adduct in the traditional lithium alanate process can lead to significant decomposition of the alane. Economically viable solutions to such problems remain elusive.
What is needed in the art is an economical method for formation of alane. For instance, a method that can utilize low cost reactants and that can provide a route for facile separation of the alane product (e.g., α-alane) from reactants and impurities would be of great benefit.